Hello everyone,
I am building a home-made vibrational circular dichroism spectrometer, and for that I use polarization modulation of my IR signal. I have a photoelastic modulator (PEM) that modulates my signal at 50kHz, and in order to retrieve the small signal carried by the 50kHz frequency, I need to demodulate it. For that purpose I use a lock-in amplifier (LIA) (Stanford SR830), but when I plug the "Ref In" of the PEM, the LIA detects a 100kHz frequency, and I specifically need to demodulate my signal at 50kHz.
Does anyone jnow why would that be ? And how I can change this 100kHz "locked" frequency to 50kHz ?
Thanks in advance,
Benjamin
What is the waveform of your PEM Ref In signal? Your LIA can use 3 types - sine, TTL +ve edge and TTL -ve edge.
Some PEMs reference output can be set to twice the modulation frequency. Have you checked with an oscilloscope or frequency meter that the signal is 50kHz?
https://www.zhinst.com/applications/principles-of-lock-in-detection
I agree with the John Miller that first you check the frequency of reference signal from PEM. Once the reference signal is at 50 K Hz, then lock in amplifier can not generate signal at 100 K Hz in any case. This reference signal governs the functioning of lock in amplifier. In fact lock in amplifier gives DC signal in its out put instead of AC signal. It will be better to go through the functioning of lock in amplifier to clarify such types of problems or confusion.
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